The present invention relates to treatment of composite polyamide reverse osmosis membranes. More specifically, it relates to the treatment of polyamide membranes, used in the purification of salt water, to enhance the flux of water through the membrane and lower the salt passage. Additionally the treatment of the membranes in accordance with the present invention improves their resistance to base exposure.
The use of reverse osmosis membranes is based on the ability of such membranes to selectively allow passage of certain components of the mixture to be separated. In the purification of salt water, such as seawater, the water is contacted with one surface of the reverse osmosis membrane under pressure to cause effective permeation of water through the membrane while minimizing the passage of the salt in the feed solution.
The efficacy of a salt water reverse osmosis membrane, although primarily determined by the combination of high flux of the water and low salt passage, also depends on its ability to withstand chemical attack from impurities or additives contained in the water. Long term exposure to chlorine is well known to cause degradation of membranes and consequent failure of such membranes in the water purification processes in which they are employed. It is also well known that the effect of chlorine on the membrane depends on the chemical structure of the membrane. This is particularly true in polyamide membranes that contain a number of different sites where chlorine can react with the polymer. The degree to which polyamide membranes are attacked depends on the particular acids and amines employed in the formation of the polyamide.
Membranes used in water purification furthermore are run under a wide range acid and base conditions, i.e., pH, and subjected to periodic cleaning under even more severe conditions. Such cleaning is conducted at high pH in order to remove biological fouling and also to free the membrane of other colloidal and particulate materials. Long-term base stability is also a very desirable property. In many applications, such as in ultra-pure water systems, it is highly desirable to operate at as high a pH as possible since at such high pH permeation of other solutes in the water, such as dissolved boron and silica containing compounds or complexes and organic carbon containing compounds, is significantly lowered.
Polyamide membranes have been widely employed in reverse osmosis processes. U.S. Pat. No. 4,277,344 ('344), the teachings of which are hereby incorporated by reference, describes a variety of membranes having a polyamide discriminating layer on a porous support. These membranes are preferably prepared by interfacial polymerization of a difunctional aromatic amine, such as meta-phenylene diamine, and a trifunctional aromatic acyl halide, such as trimesoyl chloride and 1,3,5-cyclohexane tricarbonyl chloride, optionally also containing difunctional acyl halides. Such membranes are further illustrated in U.S. Pat. Nos. 4,520,044 and 4,606,943, the teachings of which are also hereby incorporated by reference.
The treatment of membranes to enhance their performance is disclosed in a number of patents. Thus the '344 mentioned above discloses the post-treatment of the polyamide membranes with a solution of a chlorine releasing agent, e.g., hypochlorite, at a pH of 8-9, as based on the concentration of the hypochlorite used, in order to improve the performance of the membrane. U.S. Pat. No. 5,051,178 discloses the treatment of certain polyamide composite thin film membranes based on polyamides obtained by condensation involving mixtures of amines and particularly triamines, with chlorine or a chlorine generating agent at a pH of 6-13, in order to improve salt rejection and water flux. U.S. Pat. No. 4,761,234 discloses the treatment of a polyamide thin film composite membrane employing triamino benzene as one monomer with an aqueous solution containing residual chlorine at a pH of 10.3 to improve the flux of the membrane at the cost of salt rejection.
The foregoing art has demonstrated that although the treatment of a membrane with an oxidizing agent and particularly an agent releasing chlorine can improve the flux of a polyamide membrane, it is generally at the cost of some other performance characteristic of the membrane. Nothing in the art indicates recognition of the significance of the pH or basicity of any chlorine treatment on the performance characteristics of composite polyamide membranes including the stability of the resulting membrane to base. It is therefore desirable to develop treatments of polyamide membranes that result in an improvement of both flux and salt rejection. It is a further object of this invention to provide reverse osmosis membranes that can be operated at very basic conditions without suffering an increase in salt passage. It is a further object of this invention to provide membranes exhibiting superior performance with respect to impurities in water other than salt.